In an array antenna that electronically scans a main lobe in an angle range of −α to α and performs target detection, when an element-antenna distance is indicated by d, and a wavelength of transmission and reception radio waves is indicated by λ, there are cases in which a grating lobe appears in a scanning range (−α to α) of the main lobe when d>0.5λ/sin α.
When the grating lobe appears in the scanning range of the main lobe, a direction of the target is likely to be erroneously detected. For this reason, it is desirable to prevent the grating lobe from appearing in the scanning range of the main lobe.
Here, when the element-antenna distance d is reduced to be sufficiently smaller than the wavelength λ, it is possible to prevent the grating lobe from appearing in the scanning range of the main lobe regardless of a directional angle of the main lobe. However, the wavelength λ and the element-antenna distance d are restricted by various conditions, and it is difficult to set the wavelength λ and the element-antenna distance d beyond an upper limit or a lower limit of the conditions.
In this regard, a technique of removing erroneous detection caused by the grating lobe has been developed in the past.
For example, in an array antenna device disclosed in Patent Literature 1, a transmission array antenna and a reception array antenna each of which is configured such that a plurality of element antennas are arranged on a straight line at equal intervals are provided. When M and N are relatively prime integers, the element antennas of the reception array antenna are arranged near a first null occurrence angle in an array element pattern of the element antennas with a quotient obtained by multiplying the integer M by the wavelength and dividing by the first null occurrence angle so as to have an M-th grating lobe of an array factor of the reception array antenna, and the element antennas of the transmission array antenna are arranged at intervals of a quotient obtained by multiplying the integer N by the interval of the element antennas of the reception array antenna and then dividing by the integer M so that an occurrence angle of an N-th grating lobe of an array factor of the transmission array antenna is identical to an occurrence angle of the M-th grating lobe.
As described above, in the technique disclosed in Patent Literature 1, the grating lobe is suppressed by taking a product of directivity patterns of two types of array antenna element arrangements. However, since it is difficult to suppress all the grating lobes in the angle range of beam scanning only through this, the remaining grating lobes are suppressed by causing the occurrence angle of the grating lobe to be identical to an angle at which a null point at which an antenna gain is noticeably low in antenna element directivity (an element factor) is formed.